The present invention relates to a double-cantilever beam type test piece and a corrosive environmental crack growth measuring apparatus, and more particularly, to a technique of measuring growth speed of a crack such as corrosion cracking, that is, a double-cantilever beam test piece (DCB sensor) which is capable of exhibiting nearly constant stress intensity factor at crack tip when applied with a load caused due to application of a fixed displacement irrespective of crack length, and a corrosive environmental crack depth measuring apparatus which is suitable for judging the degree of corrosive environment based on the crack growth rate calculated automatically from the change of crack depth with time obtained from measurements of the crack length in the test piece placed in the actual environment by on-line measurement using a direct current potential method.
Recently, it has become clear that since structures in a boiling water type reactor (BWR) are exposed to an intensive radioactive environment, stainless steel is caused to be sensitized by neutron irradiation to give rise to a problem that there is a good chance of generation of irradiation assisted stress corrosion cracking or what is called IASCC. As a method for controlling such IASCC, it has been proposed to inject hydrogen. In this method, injection of hydrogen contributes to reduction of concentration of dissolved oxygen and control of corrosive potential. According to this method, however, it becomes necessary to observe whether the IASCC is controlled or grows within the reactor. To cope with this, as an apparatus for measuring crack growth rate within a corrosive environment, General Electric Company, U.S. has proposed an apparatus for measuring crack length by applying a fixed displacement to (DCB sensor) utilizing a potential drop method (Japanese Patent Unexamined Publication No. 62-177440). This measuring apparatus is presently applied experimentally to actual reactors.
However, although it has been determined that distribution of stress intensity factor in DCB sensor is uniform and although the test piece is seven inches in overall length, the DCB sensor of the above conventional measuring apparatus has such characteristics that, as the crack grows by one inch, the stress intensity factor is reduced to 60% of the initial stress intensity factor, and the conventional DCB sensor is not suitable for measuring the crack growth rate with a proper stress intensity factor.